The evaluation of flow-count and tetraONE alternative tehcnologies for absolute CD4 and CD8 T lymphocyte determinations are being performed in five NIAID certified laboratories from different geographic locations. Because TetraONE is a reagent/software system that operates exclusively on a coulter EPICS XL platform, only sites using coulter EPICS XL flow cytometers will be utilized. The five certified laboratories have been evaluating flow-count and tetraONE methods in a precision study measuring both the within laboratory variation and the between laboratory variation of each method and comparing the latter statistical parameters with those of the predicate method which uses flow cytometry and routine hematological counting instrumentation to determine absolute lymphocyte counts. In addition, the variation between analyses of specimens evaluated immediately (less than 6 hours old) and after standing for 24 hours at room temperature are being determined.